1. Field of the Invention
The present invention relates to a method for synthesizing semiconductor polycrystals, in particular, polycrystals of III-V or II-VI compound semiconductors and an apparatus therefor, and more particularly the present invention relates to a method for manufacturing compound semiconductor polycrystals, which makes it possible to synthesize them in a high yield and a remarkably reduce time for obtaining the polycrystals and to an apparatus efficient to carry out the process.
2. Description of the Prior Art
Recently, there has been a marked tendency, in semiconductor technology, to require semiconductor devices capable of high speed and high frequence applications. To achieve such improvement in semiconductor devices, various kinds of compound semiconductors such as gallium arsenide (GaAs) belonging to the III-V compound semiconductors, have drawn great attention, because of their high electron mobilities which help to minimize series resistances and their high saturation drift velocities which result in increased cutoff frequencies. Thus, research for improving the quality of compound semiconductor materials, for example, the III-V compound semiconductor such as gallium arsenide (GaAs), and indium phosphide (InP) and the II-VI compound semiconductor such as cadmium selenide (CdSe), the zinc sulfide (ZnS) has extensively been carried out and their quality have been improved day by day, and further devices made from such compound semiconductors have also been greatly improved in their quality.
These compound semiconductors are generally utilized to manufacture light emitting devices such as semiconductor lasers and elements receiving such light useful in the field of optical-fiber communication, field effect transistors (FET), or other sensors, and these semiconductors have been expected to permit the substantial improvement in physical qualities of these devices.
To manufacture these compound semiconductor devices, single crystals of high purity are generally required. For this purpose, various methods for preparing single crystals of compound semiconductors have been proposed such as the liquid encapsulated Czochralski technique (LEC technique), the horizontal or vertical Bridgman technique, or other improved methods thereof, and as a result single crystals having excellent properties have been obtained.
When preparing compound semiconductor single crystals, first of all it is necessary to form polycrystalline material thereof and then the polycrystalline material must be used as the starting material for obtaining single crystals. Such polycrystalline material may be prepared by reacting two different elements. However, the preparation of polycrystals of compound semiconductors is generally considered to be very difficult, since they are composed of elements having a high dissociation pressure (in other words, low decomposition temperature) , for instance, phosphorus (P) and arsenium (As) of the group V elements, cadmium (Cd) of the group II elements, sulfur (S) of the group VI elements or the like. The presence of such elements of high dissociation pressure makes the preparation procedures quite complex and it is difficult to obtain a polycrystalline compound semiconductor having precise stoichiometry.
In this connection, a method for synthesizing polycrystalline compound semiconductors will now be explained on the basis of the synthesis of an (InP) polycrystal. This is generally carried out according to the gradient freeze method which comprises encapsulating indium (In) and phosphorus as the starting elements in a quartz ampoule at desired regions spaced apart from each other, horizontally placing the ampoule in an apparatus provided with two independent heaters arranged around the apparatus in line with the longitudinal axis thereof, adjusting the power of the heaters to establish a desired temperature distribution (having a certain gradient) in the apparatus along its horizontal direction so that the temperature of the region in which the phosphorus is present becomes low, and then changing the relative position of the ampoule with respect to the heaters so that the indium reacts with the phosphorus vapour which moves toward the indium in the ampoule to form the (InP) polycrystal.
However, the crystal growth temperature of the (InP) is around 1060.degree. C. when the (InP) polycrystals are formed according to the gradient freeze method and at that temperature, the vapour pressure of phosphorus at the dissociation equilibrium is as high as 27.5 atm. and therefore, the stoichiometric polycrystals of (InP) cannot be obtained. Further, the segregation coefficient of indium during the synthesis of (InP) polycrystal largely deviates from 1 and as a result the indium content in (InP) polycrystal manufactured according to the method mentioned above is quite low at the initial stage of the polycrystal synthesis. On the contrary, the content of the indium in (InP) crystal is quite high at the final stage, thus the resulting polycrystal has a concentration gradient in which the content of indium increases along the crystal growing direction. Therefore, the extremity of the crystal in the vicinity of, the end point of the synthesis is extremely rich in indium concentration, which cannot be used as the starting material for obtaining the single crystal of (InP) and, in general, the extremity thereof is previously removed by fusing that portion prior to use as the starting material for single crystal preparation.
The method for manufacturing compound semiconductor polycrystals has now been explained referring to the case of (InP). However, the same disadvantages as those accompanied by (InP) preparation are generally encountered in preparing other compound semiconductors, in particular, those including a high vapour pressure component such as mentioned above.
Under such circumstances, there is proposed a new method for preparing polycrystals in which LEC puller is used, instead of the gradient freeze method. The apparatus has been used for preparing single crystals of compound semiconductors which comprises introducing a melt of a starting material into a crucible which is rotatively supported; disposing the crucible in a furnace in which a desired temperature distribution is established; immersing a seed crystal in the melt, which is fixed on a lower end of a rotative rod or shaft; pulling the rod while rotating the crucible and the seed crystal in opposite directions to grow single crystals. In this method, the melt of ingredient is covered with a liquid encapsulant and nitrogen gas or argon gas is introduced in the crystal growing chamber at a high pressure to prevent the escape of the component of high dissociation pressure. Thus, this method is referred to as the liquid encapsulated Czochralski (LEC) method.
The LEC growing apparatus per se is, in general, used for growing single crystals. However, the apparatus is provided with a vessel capable of bearing high pressure, heaters, upper and lower axis (shafts) which may rotate and move up and down, a susceptor or the like and the apparatus is considered to be useful to synthesize polycrystals of compound semiconductors. Thus, J. P. Farges has already proposed a method for preparing (InP) polycrystals utilizing such an LEC apparatus (See, J. Crys. Growth, 1982, 59, 665-668).
In this method, red phosphorus as the starting material for phosphorus is gasified and the resulting phosphorus vapour is bubbled into a melt of indium to react with each other. However, the phosphorus vapour bubbled in the indium melt reacts with indium only partially and the unreacted gaseous phosphorus escapes into the space of a growing furnace (or a pressure vessel) through a liquid encapsulant layer. As a result, the space of the growing furnace is filled with phosphorus vapour and it is often observed that the part of the phosphorus vapour is deposited on the wall of a pressure vessel. In addition, the deposition of phosphorus vapour is also observed on a sight through window and therefore, the control or monitor of the conditions for crystal growing cannot be effected sufficiently.
According to the inventors' experiences, the yield of (InP) polycrystal prepared by the method explained above is as low as 80% and therefore, the efficiency of this method is not so high. In other words, the amount of phosphorus required to completely convert one mole of indium to (InP) is equal to at least 1.5 moles and thus, all the indium may be recovered as (InP) if the synthesis of (InP) is carried out under the presence of phosphorus in an amount 1.5 times larger than that required to satisfy the stoichiometry. The method according to J. P. Farges is less economical, this is because a third of the amount of phosphorus does not take part in the (InP) synthesis and is discharged into the space of the pressure vessel without being consumed to form the (InP) polycrystal.
Moreover, in the method, the reaction of phosphorus with indium melt is carried out by introducing the vapour of phosphorus into the melt in the form of bubbles having a relatively large radius and as a result, the reaction area (or contact frequency between a phosphorus atom and an indium atom) is quite low, and therefore, the method needs a substantially long period of time to complete synthesis of the (InP) polycrystal and therefore, it is industrially unfavorable.